But I do not want to model 'O' as a node or eClass because it has not a semantic meaning.
The 'O'-point (or connector) is actually needed for syntactic correctness of the graphical model.
So im looking for a way to get this 'O' as decorator or so...

I'm not a GMF expert, but there is no connector like your O.
If you refer to the basic graph theory, on your scheme O is a node, as A B C and X.
Create a node O to link everything is, i think, the best solution.

The other solution should be to modifiy the diagram code to redraw A--W, B--X and C--X when you add a new connection.